In a modern paper or board machine the web transfer from the press section to the dryer section is typically carried out in a closed way without an open transfer by guiding the web at the full width until to the first dryer in the dryer section. Typically the first dryer of the dryer section is adapted to a so-called single fabric run.
The web can also be taken to the first dryer in a form of a narrow tail, which is then allowed to widen to the full width at the dryer. This is the method normally used for example in the paper machines, which have a traditional press type and an open web transfer from the press section to the dryer section. Other equipment, such as a threading blow unit or other drying unit, may be located between the press section and the first dryer. From the dryer, the web is typically allowed to travel through a doctor into a pulper located beneath the machine or to a broke conveyor or similar equipment situated below the machine.
Once the full-width web has reached the first dryer, only a narrow band of the web, a so-called tail, is first guided further to the end of the dryer section. In this system the tail is arranged to follow the fabric at least partially when travelling further in the dryer section. Once the web end has been successfully transferred to the end of the dryer section or to the end of the desired part of the dryer section, the tail is widened to the full web width so as to permit taking the full-width web through the dryer section or a desired part of it.
Unless otherwise indicated, the term ‘tail threading’ is used below to refer to both the initial stage of tail threading, during which only a narrow tail is taken through the dryer section or similar, and the tail widening stage, during which the narrow tail is widened to a full-width web.
The web is typically transferred further in the dryer section by means of a separate narrow initial band, i.e. the tail, which has been cut from the web edge and which is first guided further in the dryer section. The rest of the web forms another, an almost full-width band, which is to be “run down”, and which is not guided to the dryer section at the initial stage, but is taken at the first dryer doctor down to a pulper located under the machine or to a broke conveyor or similar equipment located under the machine.
The appropriate detachment of the various parts of the web from the dryer both during tail threading and normal operation is slightly problematic. During the normal operation the web should detach from the dryer at its full width immediately in the gap formed after the dryer and the fabric. In the tail threading situation, on the contrary, only a narrow tail or a widening tail threading band should detach from the dryer and the rest of the web, first the main part of the web, should remain attached to the dryer surface for a determined distance even after the opening gap.
Tail threading from the first dryer onwards may therefore be particularly problematic when running at high speeds, such as speeds exceeding 1300 m/min. In such conditions, the edge nearest to the tail of the band to be taken down easily tends to follow the fabric further in the dryer section, since the action directed to the tail also affects this edge and vice versa. For example, it has been noticed that in certain cases the edges tend to drift on top of each other after the cutting point, which then tears the tail edge at the final stage of the widening operation.
To be able to carry out tail threading from the first dryer onwards using the tail, it must be prevented that the aforementioned second band, i.e. the main part of the web, starts to follow the web too early causing thus problems in the dryer section, such as excessive broke accumulation in the dryer section basement. Therefore, the main part of the web must be kept attached to the first dryer surface in a controlled way during tail threading even in the gap formed by the dryer and the fabric.
The web travelling from the press section easily detaches from the first dryer surface, because the web cannot have been attached to it properly, utilizing e.g. a nip, such as takes place in the press section, where the web is attached to the surface of a smooth roll by means of a nip. If the web were too strongly attached to the dryer surface, it would result in web detachment problems at the stage when it is finally desired to detach the web from the dryer.
Efficient high-vacuum blow/suction boxes have been developed for dryer sections. These boxes can be used to make the web controllably follow the dryer fabric during normal operation after the opening gap of the dryer and fabric, even when running at high speeds. By using these boxes it can be ensured that the actual narrow tail and the web part widening at the widening stage reliably follow the dryer fabric. Breaks must not be produced in either the tail or the widening web and therefore the web transfer must take place in a controlled way. Because a very high vacuum is applied in these vacuum boxes, the web part to be run down may tend to follow the fabric in the tail threading stage instead of controllably following the first dryer and dropping down to the pulper or similar under the machine only at the dryer doctor.
International patent applications WO 98/3397 and WO 02/35001 propose some solutions for the aforementioned problems. The former publication makes known equipment, which uses two water cutters to cut a tail at the center of the web. At least one of the cutters is moved to the web from outside to ensure that the band remaining at the edge is sharp-edged and the tail is produced without an excess tail end. This, however, does not resolve the problems at the widening stage.
The latter publication is even more closely related to the present invention. It discusses the aforementioned problem of arranging an appropriate detachment of the various web parts at each stage. It is attempted to prevent the edge nearest to the tail from following along by providing adjustable suction in the cross-machine direction on the outlet side of the dryer. The transfer zone of the vacuum box suction is synchronized with the spreader slide movement. In addition, a narrow intermediate band is formed between the actual tail and the web part running to the pulper. The intermediate band may be simply a wider cut produced with high pressure and a large amount of water. These solutions have disadvantages. A third band, leaving in an unspecific way, can after all cause problems later in the machine. A cut made applying a higher pressure wets and soils the fabric more than usually.